A thin client system is a system in which a client terminal that a user uses (referred to as a user terminal, hereinafter) performs the minimum necessary processes while most processes are carried out by a server. In recent years, in order to reduce the risk that information would be leaked from stolen PCs (Personal Computers), or in order to reduce costs in managing the user terminals, the thin client system is focused on. In the thin client system, the user terminals do not hold data. Therefore, if the user terminal is stolen, the likelihood that important data such as personal information would leak is low. Moreover, in the thin client system, OS (operating system) and applications are centrally controlled by the server. Therefore, the total management cost is lower than when the individual user terminals are managed. One example of such a thin client system is described in Non-Patent Document 1.
Non-Patent Document 1 describes three types of thin client system: a network boot type (a network-boot-type), a screen transfer type, and a virtual PC type.
The network boot type is a system in which OS files, application files, and data files that the user terminals use are put together in the server. According to the system, OS and applications are loaded from a hard disk of the server onto the user terminal which then executes OS and the applications. Data are loaded from the hard disk of the server onto a memory of the user terminal by the applications performed on the user terminal. The system can endure even when a network connection is cut off because the user terminal does not use the network after OS, applications, and data are loaded onto the user terminal.
The screen transfer type is a system in which the server executes applications and then transfers screen information to the user terminal. According to the system, at least a program for displaying screens runs on the user terminal; a special-purpose processor for displaying screens may be used. Data are read by applications running on the server. According to the system, information leaks rarely occur because data are not loaded onto the user terminal.
The virtual PC type is a system in which a virtual machine for client runs on the server, with applications performed on the virtual machine to transfer image information to the terminal. The virtual PC type is substantially the same as the screen transfer type except that the virtual machine is used instead of the server. That is, OS and a program for displaying screens run on the user terminal. Data are read by applications running on the virtual machine. According to the system, like that of the screen transfer type, information leaks rarely occur. Accordingly, the virtual PC type can be considered to be one form of the screen transfer type.
The network-boot-type is available on PC equipped with a network boot function called PXE (Preboot eXecution Environment) even if PC does not have OS and applications installed. Meanwhile, what the screen transfer type needs to do is just to display screens. Therefore, high-speed CPU and large-capacity memories are unnecessary, making it possible for cellular phones to be screen-transfer-type user terminals for example.
Patent Document 1 describes other example of the thin client system. According to the thin client system disclosed in Patent Document 1, a plurality of thin client programs can be stored in the user terminal. The programs can be selected at the time of startup. Therefore, one user terminal (a network terminal device) can serve as a thin client for a plurality of servers.
FIG. 12 is a block diagram illustrating the configuration of a thin client system disclosed in Patent Document 1. With reference to FIG. 12, basic software is stored in user terminals 905a to 905n to use Windows NT (Registered Trademark) server 902 and UNIX (Registered Trademark) server 903, and Terminal server 904. The basic software is software that is stored in the user terminals and is used by the user terminals. The basic software is the minimum necessary software, such as one type of OS or software for displaying, to remotely control various kinds of application software stored in the server via a network 901. Each of the user terminals 905a to 905n is configured so that a plurality of different basic software programs can be stored.
When a user selects a server that the user wants to use during the startup process of the user terminal, a basic software corresponding to the server is selected and starts running. Then, the user terminal is connected to the server via the network 901. Once the server is selected, the previously-used server will be selected as default. The basic software stored in the user terminal can be changed by downloading. When the basic software is changed, the user selects a storage device where the program will be saved on the user terminal side. Then, the server is operated to transmit the basic software. Moreover, a plurality of the servers may be put together into physically one server. According to such a configuration, the user can switch the basic thin-client software to use a plurality of thin-client servers. Furthermore, a relatively small OS that does not use the thin-client servers may be used.    Patent Document 1: JP-A-2000-339245 (Pages 3 to 8, FIG. 1)    Non-Patent Document 1: NEC Web page, “Thin Client System,” [online], [Searched on Feb. 16, 2007] Internet <URL: http://www.express.nec.co.jp/products/thinclient.html>